<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML>
<HEAD>
    <TITLE>Peekable</TITLE>
    <LINK REL="stylesheet" HREF="../../../../boost.css">
    <LINK REL="stylesheet" HREF="../theme/iostreams.css">
</HEAD>
<BODY>

<!-- Begin Banner -->

    <H1 CLASS="title">Peekable</H1>
    <HR CLASS="banner">

<!-- End Banner -->

<H2>Description</H2>

<P>
    A Peekable <A HREF="device.html">Device</A> allows a character read from the input sequence to be putback. 
</P>

<P>
    The name of the concept is based on the observation that a PeekableDevice allow the user to peek at the first character in the sequence it controls by calling <A HREF="../functions/get.html"><CODE>get</CODE></A> or <A HREF="../functions/read.html"><CODE>read</CODE></A> followed by <A HREF="../functions/putback.html"><CODE>putback</CODE></A>. The names Putbackable and InputRevertable were considered but rejected.
</P>

<H2>Refinement of</H2>

<A HREF="source.html">Source</A>.

<A NAME="types"></A>
<H2>Associated Types</H2>

Same as <A HREF="source.html">Source</A>.

<H2>Notation</H2>

<TABLE CELLPADDING="2">
    <TR><TD><CODE>P</CODE></TD><TD>-</TD><TD>A type which is a model of Peekable</TD></TR>
    <TR><TD><CODE>p</CODE></TD><TD>-</TD><TD>Object of type <CODE>P</CODE></TD></TR>
    <TR><TD><CODE>c</CODE></TD><TD>-</TD><TD>Object of type equal to the character type of <CODE>P</CODE></TD></TR>
    <TR><TD><CODE>io</CODE></TD><TD>-</TD><TD>Alias for namespace <CODE>boost::iostreams</CODE></TD></TR>
</TABLE>

<H2>Valid Expressions / Semantics</H2>

<TABLE CELLPADDING="5" BORDER="1">
    <TR><TH>Expression</TH><TH>Expression Type</TH><TH>Precondition</TH><TH>Semantics</TH><TH>Postcondition</TH></TR>
    <TR>
        <TD>
            <PRE CLASS="plain_code"><CODE><A HREF="../functions/putback.html">io::putback</A>(p, c)</CODE></PRE>
        </TD>
        <TD><CODE>bool</CODE></TD>
        <TD>
           One or more characters has been read from <CODE>p</CODE> using <A HREF="../functions/get.html"><CODE>io::get</CODE></A> or <A HREF="../functions/read.html"><CODE>io::read</CODE></A>, with no intervening i/o operations having been performed
        </TD>
        <TD>
            Attempts to put back <CODE>c</CODE> to <CODE>p</CODE>, returning <CODE>true</CODE> for success
        </TD>
        <TD>
            If putback returns <CODE>true</CODE>, the next character read from <CODE>p</CODE> using <A HREF="../functions/get.html"><CODE>io::get</CODE></A> or <A HREF="../functions/read.html"><CODE>io::read</CODE></A> will be equal to <CODE>c</CODE></A>
        </TD>
    </TR>
</TABLE>

<H2>Exceptions</H2>

<P>
    Errors which occur during the execution of <A CLASS="code" HREF="../functions/putback.html"><CODE>putback</CODE></A> are be indicated by throwing exceptions.
</P>

<H2>Models</H2>
<UL>
    <LI>Standard input streams and stream buffers.
</UL>

<!-- Begin Footer -->

<HR>

<P CLASS="copyright">&copy; Copyright 2008 <a href="http://www.coderage.com/" target="_top">CodeRage, LLC</a><br/>&copy; Copyright 2004-2007 <a href="https://www.boost.org/users/people/jonathan_turkanis.html" target="_top">Jonathan Turkanis</a></P>
<P CLASS="copyright"> 
    Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <A HREF="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>)
</P>

<!-- End Footer -->

</BODY>